Design Question: Office 365 + Exchange 2016 Hybrid

Hello All!

I was wondering if any expert minds had a better way to do this. We want to have this setup in our environment:

The only issue is that the amount of IPs required to pass through the firewall under Exchange Online at this URL is quite large. Adding it to both the connector on our on-prem exchange server and firewall makes it extremely messy.

The reason we desire this flow is for our signature software (CodeTwo Exchange) that stamps everyone's outbound e-mail with the standard signature. We also have journaling but that doesn't affect it either way.

We currently have it setup so that the Office 365 mail is going right to Proofpoint (as a test). It makes firewall rules+connector whitelisting much easier but there are issues with internal e-mails being tagged as phishing e-mails + signature issues.

Ok, so you want Incoming mail to go through Proofpoint, then on-prem, then cloud, correct? And Outgoing should do the reverse?

You'll first want to run through the Hybrid Configuration Wizard tool that is available from MS through O365 to set up the relationship between the onprem server and Office 365. That will actually create the connectors necessary to communicate with O365 along with all the necessary IPs. In the wizard you would just configure things using the option to have all mail go through your On-prem environment.

The issue with Office 365 messages getting flagged as spam through Proofpoint is actually kind of tricky to resolve, since it would require you to white-list either all O365 IPs or configure it so your email domain is acceptable in the From: field for incoming mail, both of which significantly increase the amount of spam you'll get, since any spam sent from a different O365 tenant would end up getting allowed, and using your address in From: is a common spam tactic.

You'll also need to disable all spam filtering in Exchange Online. If you have your mail-flow going through Proofpoint, you'll be double filtering messages, which is just a huge problem. Getting rid of Proofpoint would simplify things, since Exchange Online does have some fairly capable Spam/malware filters already, but if proofpoint has features you require, you can of course continue doing things through their systems.

I did forget to mention that we did run the O365 Hybrid Config but had to modify the outbound connector rules to directly go to Proofpoint for the O365 environment. Incoming e-mails actually flow perfectly.

We do want to keep Proofpoint since we also utilize the Archiving function from Proofpoint.

I did think about whitelisting O365 IPs and just getting over it but the increase in spam is also a huge concern for us.

That's what we were thinking as well -- CodeTwo does offer a nice solution. The only issue is Proofpoint quarantines internal e-mails going from O365 <=> Exchange. Not sure if there's anything around this without disabling the spoofing feature (which we do get a lot).